1. Field of the Invention
The present invention relates to a determination apparatus that determines a type of a recording medium and an image forming apparatus that mounts the determination apparatus therein, and more specifically to the determination apparatus that determines grammage of the recording medium by irradiating an ultrasonic wave to the recording medium and detecting the ultrasonic wave transmitted through the recording medium and the image forming apparatus such as a copy machine and a laser printer that variably controls an image forming condition by using a determination result of the determination apparatus.
2. Description of the Related Art
An image forming apparatus such as a copy machine and a laser printer includes an image bearing member as an image forming unit, a development unit, a transfer unit, and a fixing unit. Each unit has a function as follows.
The image bearing member is a photosensitive drum including, for example, a photosensitive layer on which an electrostatic latent image is formed. The electrostatic latent image is formed, for example, by exposing the image bearing member with a laser beam.
Further, a development unit has a function for applying developer to the electrostatic latent image formed on the image bearing member to visualize the electrostatic latent image. The developing unit can use, for example, a development roller.
Further, as the transfer unit, for example, a transfer roller is used and the transfer unit has a function for transferring the developer image to the recording medium to be carried. Furthermore, the fixing unit includes a heating roller and a pressure roller.
The fixing unit has a function for heating and pressing by the heating roller and the pressure roller the recording medium on which the developer image is transferred by the transfer roller in order to fix the developer image onto the recording medium.
In the conventional image forming apparatuses, for example, a user sets various settings by a computer as an external apparatus. Or, the user sets a size and a type (hereinafter, referred to as a “paper type”) of the recording medium using an operation panel provided on a main body of the image forming apparatus.
According to the settings, for example, the image forming apparatus is controlled to set a transfer condition (for example, a transfer voltage or a conveyance speed of the recording medium when transferred) or a fixing condition (for example, a fixing temperature, or a conveyance speed of the recording medium when fixed).
In order to decrease the user's burdens for setting the conditions via the computer or the operation panel, in recent years, there is provided an image forming apparatus including a determination sensor as a determination unit to allow the image forming apparatus to have a function for automatically determining the type of the recording medium.
Such an apparatus can automatically determine the type of the recording medium and set the transfer condition and the fixing condition described above according to the determination result.
More specifically, Japanese Patent Application Laid-Open No. 2001-139189 discusses an apparatus that determines a thickness by providing a light emitting source such as alight emitting diode (LED) at a position opposing a sensor and detecting the light (intensity of the transmitted light) that has been transmitted through the recording medium.
Further, Japanese Patent Application Laid-Open No. 57-132055 discusses an apparatus that determines grammage (weight per unit area) of the recording medium by irradiating the ultrasonic wave to the recording medium and detecting transmittance of the ultrasonic wave transmitted through the recording medium.
As discussed in the above Japanese Patent Application Laid-Open No. 57-132055, when measuring the grammage of the recording medium by using the ultrasonic wave, it is necessary to consider influences of an interference of the ultrasonic wave between an ultrasonic transmission unit (hereinafter, referred to as a “transmission unit” and an ultrasonic wave reception unit (hereinafter, referred to as a “reception unit”) and a reflection wave of the ultrasonic wave generated between the transmission unit and the recording medium or between the recording medium and the reception unit.
Further, when an ultrasonic wave sensor is applied to the above-described image forming apparatus, since the ultrasonic wave is reflected by a conveyance path for conveying the recording medium and a member such as a conveyance roller, it is also necessary to consider influence by the reflection wave.
For example, as discussed in Japanese Patent Application Laid-Open No. 57-132055, as a method for decreasing these influences, there is proposed a method for ending a measurement before a first interference of the reflected ultrasonic wave emitted from the transmission unit reaches the reception unit. The transfer time of the ultrasonic wave between the transmission unit and the reception unit has been previously calculated.
Further, as another method for decreasing the influences of the reflected wave, as discussed in Japanese Patent Application Laid-Open No. 2001-351141, the transmission unit and the reception unit are disposed obliquely with respect to the conveyance path to prevent the measurement from the influence of the ultrasonic wave reflected between the transmission unit and the recording medium or between the recording medium and the reception unit.
Furthermore, as discussed in Japanese Patent Application Laid-Open No. 2005-082350, there is proposed a method for decreasing the ultrasonic wave reflected by a peripheral member by disposing an acoustic absorbent (guide) at a periphery of the transmission unit and the reception unit.
In recent years, since high print quality has been increasingly demanded, it is necessary to form an image on various types of recording media used by a user without decreasing the print quality. More specifically, it is preferable to determine the type of the recording medium more accurately and form the image depending on the type thereof.
Particularly, in order to accurately detect the grammage of the recording medium, a method for detecting the grammage of the recording medium by using the ultrasonic wave is effective.
For a detection method by the ultrasonic wave, it is preferable that other member does not exist at a periphery of the sensor and also an environment at the periphery of the sensor is maintained under a predetermined condition. It is because the level of the ultrasonic wave, which is reflected by the other member, and received and detected by the sensor, varies. As a result, the level of the detected ultrasonic wave varies due to the variation of the environment caused by the reflected ultrasonic wave.
However, if the ultrasonic sensor is applied to the image forming apparatus, it is difficult to maintain the state and the environment of the periphery of the sensor in a predetermined condition due to the following situations.
Firstly, when the recording medium is carried, the recording medium is not always in a steady position. That is, the recording medium vibrates when carried. This is generally referred to as up-and-down movements of the recording medium. The up-and-down movements cause the recording medium to be vibrated, bended, and tilted in a vertical direction with respect to a conveyance direction.
The recording medium is rarely conveyed in the same attitude and the same position. The amount of up-and-down movements varies every time the recording medium is conveyed. As a result, a distance between the transmission unit and the recording medium and between the recording medium and the reception unit may be changed. Therefore, since the level of the signal received by the reception unit varies, it may be difficult to detect the received signal accurately.
Further, the environment where the image forming apparatus with the sensor therein is set does not always have a constant temperature, humidity, or atmospheric pressure. For example, when the ambient environment does not have a normal temperature or a normal humidity, a propagation speed in the air varies depending on the environment such as a low temperature, a low humidity, a high temperature, or a high humidity.
Therefore, when the ultrasonic wave is detected at the same timing as detected in the normal temperature or a normal humidity, the level (voltage) of the received signal may be changed. Further, since amplitude of the transmitted signal from the transmission unit is changed due to the variation of the atmospheric pressure in addition to the temperature and the humidity, the level of the received signal may be changed as well.
Furthermore, a variety of members to be used for forming images exist at the periphery of the sensor. The ultrasonic wave is reflected by the peripheral members of the transmission unit and the reception unit, and the sensor is influenced by the reflected ultrasonic wave (can be interfered).
For example, the signal acquired at a stage where the level of the signal becomes stable at a certain value after the ultrasonic signal is transmitted includes the reflected ultrasonic wave and, thus, does not have a correct level.
For example, Japanese Patent Application Laid-Open No. 57-132055 as described above discusses a method for receiving the ultrasonic wave without being influenced by the reflection wave of the ultrasonic wave. Therefore, the transmission time of the ultrasonic wave from the transmission unit to the reception unit is measured without the storage medium placed, and the grammage of the recording medium is determined based on a signal received by the reception unit after the measured transmission time has elapsed with the recording medium placed.
More specifically, Japanese Patent Application Laid-Open No. 57-132055 defines the transmission time of the ultrasonic wave as a time from a start of driving the transmission unit to a rising of a waveform of the output signal received by the reception unit. However, the rising of the output signal waveform of the ultrasonic wave signal varies according to a variation of an ambient environment such as the temperature, the humidity, and the atmospheric pressure. Accordingly, the above-described transmission time changes.
In order to correct or cancel an amount of the variation, the propagation time of the ultrasonic wave and the recording medium need to be measured alternately and frequently. However, when the propagation time is measured frequently for determining the recording medium, a procedure for determination becomes very complicated and takes time.
Further, when the recording medium is placed between the transmission unit and the reception unit for respectively transmitting and receiving the ultrasonic wave, the recording medium causes attenuation of the ultrasonic wave. Japanese Patent Application Laid-Open No. 57-132055 discusses a method for detecting an output in one cycle of the received signal waveform from a beginning of the measurement. Therefore, for example, a sufficient output for the recording medium having large grammage may not be acquired, since the output for the first several cycles of the received signal are extremely small.